Concrete answers for the cold weather quandary
Kent D. Dvorak, P.E.
Manager, Construction Services Department
The need to place concrete for public works projects does not end with the onset of winter. Whether for repairs or to finish the last project of the year there are always concrete placements that occur in less than ideal conditions. Placement of concrete in cold weather conditions has become a routine construction activity. This has been possible by new practices and material developments by private and public research. State transportation departments, the Army Corps of Engineers, and Scandinavian counties in Europe are leaders in cold weather concrete placement practices.
Low temperatures have traditionally restricted the placement of concrete in seasonally colder areas, such as the northern United States. As temperatures drop and heat is lost, freshly placed concrete sets more slowly, gains strength less rapidly, and can take as much as one or two additional hours to finish. When temperatures drop below zero, the final product can be severely damaged and compromised due to the freezing of the mix water.
The placement of concrete in cold weather is addressed in American Concrete Institute (ACI) 306, Cold Weather Concreting, and this document is routinely cited in project specifications. According to the standard, concrete can resist a freezing event when it has attained a compressive strength of 3447.4 kPa (500 psi). During the summer, when air temperatures are in the 21 C to 32 C range, this compressive strength can be attained in as little as 12 hours.
When concrete freezes before the mixture reaches the minimum strength of 500psi, the matrix of the Portland cement concrete may experience irreparable internal fracturing; this reduces the strength of the mixture. When concrete is placed in late fall or winter, it must reach an even greater compressive strength (24131.7 kPa [3500 psi] per ACI 306) to protect itself from the damage caused by multiple freeze-thaw cycles.
The placement of Portland cement concrete in subfreezing temperatures has significantly increased in all areas of construction. Methods of concrete placement and curing to compete with areas having more favorable weather have been developed. The end result is that accelerating admixtures are more attractive; their high cost no longer an impediment to their use.
Portland cement hardens by the process of hydration, a reaction driven by both ambient and internally generated heat. The higher the ambient temperature, the faster the reaction takes place. Therefore, when concrete is to be placed during cold weather it is vital to protect the mixture in the structure until the desired strength and durability characteristics are achieved.
ACI 306 defines cold weather as a three-day period where the following conditions exist:
The goal in placing concrete in cold weather conditions is to maintain an adequate mix temperature so the hydration process continues until the desired compressive strength is attained. When the concrete's temperature falls below 10 C, the hydration process slows to the point that strength gain may adversely affect construction schedules.
ACI 306 provides minimum temperatures at which the concrete must be delivered to the job site (Table 1). The traditional approach has been to maintain the required temperatures until the desired compressive strength is attained through either insulation blankets or heated enclosures. Since they typically require the least amount of manpower and equipment needs, insulated blankets are commonly used when temperatures are near or above freezing during the day. During periods of extended subfreezing weather, heated enclosures may be constructed, but this can be a costly procedure, requiring significantly more labor and materials. Both of these methods are not practical for pavement placements.
In addition to this traditional approach, accelerating admixtures and alternate strength estimating techniques such as the maturity method are being used to speed cement hydration and evaluate compressive strength.
Admixture suppliers have developed non-chloride accelerating admixtures that allow fresh concrete to develop acceptable strength when cured in cold environments. These antifreeze admixtures are able to work at temperatures below the freezing point of water. Using a combination of accelerating admixtures and winter protection procedures, concrete can be placed at any temperature.
For many years, calcium chloride was the admixture of choice for accelerating concrete strength gains. Where non-reinforced concrete is being placed, it is still the most cost-effective material available. However, it also comes with certain disadvantages. In exposed concrete structures that also contain reinforcing steel, the chlorides introduced into the mixture accelerate the metal's corrosion. Concrete mixtures containing calcium chloride are also susceptible to flash set, present a false set to flat work finishers, and cause discoloration of the concrete. For these reasons, calcium chloride's use is disallowed by most public works agencies.
Non-chloride accelerating admixtures are gaining in popularity. These admixtures typically contain high nitrates, which accelerate the cement's hydration. This acceleration increases the heat produced by the reaction, thus maintaining the temperature within the concrete mass. Although the cost of the admixture is high, the savings in labor and construction time offset the additional cost of the non-chloride accelerator. However, even with the use of these accelerators, adequate protection from the weather is still important. In the winter, plastic shrinkage cracking can be a concern when low water/cement (W/C) ratio concrete mixtures are exposed to rapid drying in low humidity and high wind environments.
Available technology, used with an appropriate cold weather concreting plan and budget, make keeping projects on schedule possible despite the cold front.
Much of the research in cold weather concrete placement in the United States is being promoted by the U.S. Army Corps of Engineers (USACE) through the Cold Regions Research and Engineering Laboratory (CRREL). Commercialization of this technology has been hindered by a lack of industry-wide standards, but the Civil Engineering Research Foundation (CERF) has spearheaded an effort to develop national standards for cold weather admixtures.
In most commercial construction projects, protecting the concrete from freezing is only part of meeting a placement schedule. This requires knowledge of the material's place strength, which dictates the placing of concrete for the next floor or for pulling forms. Slow strength development can cause significant delays in the established pour schedule. Research has shown there is a relationship between concrete strength and temperature history of the mix, and there is commercially available equipment that continuously records temperatures of the material using thermal couplers placed in the concrete and then calculates a 'maturity factor. A relationship between the maturity factor and concrete strength can be developed using standard test cylinders. Using the maturity method, the mix performance and protection procedures can be evaluated, while the materials and practices are fine-tuned to meet the desired schedule.
This sort of careful planning must be discussed by all members of the design/construction team. Thanks to the proper use of available technologies, cold weather concreting can be done efficiently while keeping projects on schedule and budget. However, different concrete placement situations require different approaches in cold weather—consideration must be given to whether the concrete will be poured below, on, or above grade. Additionally, an appropriate cold weather concrete plan and budget should be considered as part of a pre-placement meeting, rather than a last-minute decision made when the cold front moves in.
Kent D. Dvorak, P.E., has worked in the construction materials field for over 25 years and was recently named Engineer of the Year by the Kansas Society of Professional Engineers. A graduate of Iowa State University, he is currently the Construction Services Department Manager for the Lenexa, Kansas, office of Terracon, a consulting engineering firm providing geotechnical, environmental, and construction materials engineering services. Dvorak can be contacted via e-mail at firstname.lastname@example.org.
Tips for Cold Weather Concreting